An international study involving more than 40,000 volunteers will examine the benefits of the mineral selenium.

The results of the study will fuel the debate over the usefulness of taking daily supplements.

At the moment, there is not enough evidence to support the idea that dietary supplements can prevent cancer.

However, following US research suggesting there were health benefits for those who increased their daily intake of the mineral, the Cancer Research Campaign (CRC) decided to launch the study.

What the mineral does

Selenium is a micronutrient that has an antioxidant effect when combined with vitamin E.

Antioxidants have been shown to protect against a wide spectrum of diseases.

Selenium was discovered in 1817 and takes its name from the Greek for moon because of its pale colour.

High levels can be found in meat, fish, grain and Brazil nuts.

However, selenium levels in grain depend on the soil where it is grown.

In the US, there are naturally high levels of the mineral in the soil, while in Finland supplements in are added to fertilisers to raise levels.

The US study showed that people who took 200 micrograms of selenium a day halved their risk of getting cancer.

The researchers said people taking the extra dosage had 63% fewer prostate cancers, 58% fewer colorectal cancers and 46% fewer lung cancers.

How the trial will work

Dr Margaret Rayman, a nutrition specialist at Surrey University, will lead the CRC study.

"The trial aims to show not only whether selenium has a protective effect against cancer, but also how much is needed to have this effect and which people will benefit most."

In total, the trial will last about seven years, consisting of a two-year pilot and a five-year main study.

The participants will get either a 100 microgram, a 200mcg or a 300mcg pill or a dummy pill with no selenium each day.

The researchers will follow their progress to see if a link between supplements and better health can be supported.